DEV: Expose AI spam scanning metrics (#1077)

This should give us a better idea on how our scanner is faring across sites.

```
# HELP discourse_discourse_ai_spam_detection AI spam scanning statistics
# TYPE discourse_discourse_ai_spam_detection counter
discourse_discourse_ai_spam_detection{db="default",type="scanned"} 16
discourse_discourse_ai_spam_detection{db="default",type="is_spam"} 7
discourse_discourse_ai_spam_detection{db="default",type="false_positive"} 1
discourse_discourse_ai_spam_detection{db="default",type="false_negative"} 2
```

.rubocop.yml

@@ -2,3 +2,6 @@ inherit_gem:
   rubocop-discourse: stree-compat.yml
 RSpec/NamedSubject:
   Enabled: false
+
+Style/GlobalVars:
+  AllowedVariables: [$prometheus_client]

about.json

@@ -0,0 +1,7 @@
+{
+  "tests": {
+    "requiredPlugins": [
+      "https://github.com/discourse/discourse-prometheus"
+    ]
+  }
+}

lib/ai_moderation/spam_metric.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module DiscourseAi
+  module AiModeration
+    class SpamMetric
+      def self.update(new_status, reviewable)
+        ai_spam_log = AiSpamLog.find_by(reviewable:)
+        return if ai_spam_log.nil?
+
+        increment("scanned")
+        increment("is_spam") if new_status == :approved && ai_spam_log.is_spam
+        increment("false_positive") if new_status == :rejected && ai_spam_log.is_spam
+        increment("false_negative") if new_status == :rejected && !ai_spam_log.is_spam
+      end
+
+      private
+
+      def self.increment(type, value = 1)
+        metric = ::DiscoursePrometheus::InternalMetric::Custom.new
+        metric.name = "discourse_ai_spam_detection"
+        metric.type = "Counter"
+        metric.description = "AI spam scanning statistics"
+        metric.labels = { db: RailsMultisite::ConnectionManagement.current_db, type: }
+        metric.value = value
+        $prometheus_client.send_json(metric.to_h)
+      end
+    end
+  end
+end

plugin.rb

@@ -84,6 +84,9 @@ after_initialize do
 
   on(:reviewable_transitioned_to) do |new_status, reviewable|
     ModelAccuracy.adjust_model_accuracy(new_status, reviewable)
+    if DiscourseAi::AiModeration::SpamScanner.enabled?
+      DiscourseAi::AiModeration::SpamMetric.update(new_status, reviewable)
+    end
   end
 
   if Rails.env.test?